Method of strengthening tubular metallic objects



April 6, 1943.

H. E. soMEs 2,315,558

METHOD OF STRENGTHENING TUBULAR METALLIC OBJECTS Filed April ll, 1940 lu@iinrlmmnnnnnmnllmmummugmiiif rr,mmmqyumnu|lmmmmmlgmgnfm1u@i1 IIIIII nl@IlHIIIIIIIIIIIIWEHIHIIIII INVENTOR Howard E. Somes A TTURNE Y Patented Apr. 6, 19;,13

UNITED STATES EATENT oFFIcE METHOD F STREN GTHENIN G TUBULAR ALLIC OBJECTS Howard E.

Somes, Detroit, Mich., assigner to Budd Induction Heating, flnc., Philadelphia, 'Ia., a-corporation of Michigan Application Aprilia 1940, serial No. :529,053 4 claims. (ci. 14s-1s) This invention relates to tubular structures and nal loading.

It has been heretofore the practice in tubular objects of homogeneous uniforn strength matetension stresses existing in the outer layers of the material. In fact, in the most desirable condition under the maximum desired and still remain within the yield point of the material.

To provide residual or trapped stresses in a tubular object having zones of non-uniform yield strength, such that the tubular object will have a maximum strength against internal loading Withventio'nwill appear more fully hereinafter from the following detailed description when taken in conjunction with the accompanying drawing. It is expressly understood, however, that the drawing is employed for purposes of illustration only and is not designed as a definition of the limits of the invention, reference being had for this purpose to the appended claims.

In the drawing wherein like reference numerals Patent No. 2,281,331, and apparatus for hardening an internal layer structure by generating heat in an structure through electrothe hardened layer and at the same time increase the stress of the portion under tension and carry the same to a higher tension stress. It will also appear that the tension stress of the outer layers will be raised to the yield point before the compressive stress of the internal layer is neutralized and changed to a tension stress. If the inner layer under no circumstances ever carries a tensile stress, it lacks any usefulness in withstanding internal loading.

Since the unit yield strength of the hardened layer under such circumstances is many times the yield strength of the unhardened portion of the tubular structure, the internal layer in order to produce a structure in which the greatest strength against internal loading is present should be initially under tension and the remaining metal initially under compression when not internally loaded. Referring to the drawing, there is illustrated a section through the tubular structure I in which an internal layer l2 has been hardened to a depth of approximately a tenth of an inch and in which the vouter layer Il remains in its original unhardened state. The relative thickness of the hardened layer to the unhardened layers may be in the ratio of one to two, and in such a structure initially the internal hardened layer resulting from the electro-magnetic induction heat treatment will be found to be under a residual or trapped comprcssive stress.

In order to reverse the trapped or residual stresses and place the hardened layer under initial tension and the outer layers under initial compression which is an object of the present invention, the internal layer may be heated substantially instantaneously to cause expansion thereof and because of the superior strength thereof, will thereupon cause the outer layers to bestressed beyond their yield point and thus be upset so that upon cooling of the tubular structure to a uniform temperature throughout, the expanded outer layers will become placed under compression and the internal hardened layer be placed under tension. In accordance with the invention, an electro-magnetic induction heating coil, such as illustrated at IB, may be passed through the length of the tubular structure so as to heat the inner layer to the desired degree. Preferably the coil I6 will be connected to a high frequency high power source of alternating current and will have threaded through the center thereof an iron core I8 in orderfto assist in the concentration of energy at a high rate in the layer in which the heat is to be generated. The construction of the coil may be substantially in accordance with that disclosed in the application Serial No. 164,320 hereinabove referred to.

By substantially instantaneously raising the temperature of the internally hardened layer of the tubular structure to the desired temperature for example such as 400 F. and applying the heat to the layer almost instantaneously and before any consequential heat may escape therefrom to the outer layers, the inner layer which was initially under compression is forced to expand because of the increase in compression due to thermal expansion resulting from the heat generated and consequent temperature rise in the internal layer. The additional compressive stress developed in the internally hardened layer through the heating thereof will be sufficient to stress the outer unhardened and cold layers beyond their elastic limit s0 that they will take a permanent set.

In practice, tubular structures treated in the manner set forth will be found to have had their actual internal diameter enlarged slightly through the treatment, and this because the external layers have been given a permanent set and stressed beyond their yield point allowing the internal layer, formerly under compression, to expand. No quenchingneed be resorted to for the structural change in the tubular object takes place immediately upon the development of heat and the raising of the compressive stresses inthe internal layer sufficiently to force the outer layers to yield, and the structure can thereafter be permitted to cool in air or if desired for rapidity, the same may be cooled by placing in a cooling medium or cooling bath or even by quenching internally alone as described in the afore-mentioned application. The structural change may be effected progressively throughout the length of a long tubular object whose internal layer has been previously hardened or if desired, by the use of a coil of suiiicient length, the entire object may have the structural changes in the stresses effected simultaneously. While a repetition of the upsetting operation employing the same heating would seem from the foregoing to effect no change over and above the change effected by the irst heating, it has been found that repetition even with the same heat and same conditions, except that the piece has already undergone physical upset from the first heat, may and does effect a reorganization of the stressed portion in the transition zone between the hardened and unhardened zones and beneficially effects the tubular structure.

It will appear that the heating of the internal layer to a temperature of for example 400 F. may not effect any adverse material change in its hardness and yet the treatment hereinabove described will materially increase the internal load strength of the tube. In some instances it may be desirable to carry the temperature created in the internal layer to a point such as produces a certain amount of drawing of the original hardness in the internal layer, in which case the internal layer may be initially hardened to a much higher degree than that flnally desired so that the drawing action will produce an amount of reduction in the hardness of the internal layer such as will produce the hardness desired.

In some instances it is often desirable to reduce the extreme hardness of such an internal layer somewhat in order to produce a better physical condition and in such cases the procedure above-referred to not only results in an improved strength against internal loading but does not materially impair the wear or corrosion resistant surface.

While a temperature of 400 F. has been suggested, the internal layer obviously may be heated to any temperature below the critical hardening temperature. The temperature chosen, of course, will be that which is necessary to produce the desired or necessary upset in the outer layers. In some instances the temperature so chosen will be limited due to a Vdesire to refrain from drawing the hardness of the hardened layer. In such cases, the temperature differential may be increased by refrigerating the work before heating the internal layer. The entire structure may have its physical characteristics further improved by subjecting it to a low temperature which 1 may be'forexample 250L7 F. lbr a suitable time such as sev'ral Hours. "e The ratio of the thickness ofthe hardened layer to that of the unhardened layer in the example given has been substantially onel totwo and it will readily appear that, for example. in a three inch tube.

hardened layer is over two -times that of the unhardened layer, the hardened layer when heated and caused to expand will have the necessary strength to stretch the unhardened layer beyond the elastic limit. The thickness of the un- `hardened layer must be such that the total strength of the hardened layer at the temperature to which it is to be raised is sulcient to overcome the yield strength of the unhardened layer, without the hardened layer reaching its elastic limit. Thus with a hardness ratio of one to five and a thin walled cylinder having a suilicient radius so that complications resulting from short radius and thick walls are substantially eliminated, it

will appear that the hardened layer should have a thickness of and above one-fifth of the thickness of the unhardened layer. Suflicient margin of safety must be left between these values so that in general practice a ratio of two or three to one will usually be found to be satisfactory although a ratio as high as four to one may be employed.-

While the invention has been described inconi junction with increasing the internal loading`r strength of tubular structures, it will, of course. appear obvious that the axial tensile strength of such a tubular object will be materially increased by such treatment. For example, the compressive stress of the internal layer prior to treatment exerts itself not only circumferentially but elementally of the tubular object and consequently. when a tubular structure of this kind is placed under axial tension, the outer unhardened layers being already under considerable tensiontin opposition to the compressive stress of the internal l-la-yer would be prevented from resisting such axial tension stress by the amount of its residual or trapped tension stress. By placing the outer layers under compression initially and the internal layers under tension,theinternal layers and external layers may, when placed under tension, combine and assist one another in resisting such a tension stress and thus the axial tension strength. of such a tubular structure is increased.

It will thus appear that the invention may be applied to at platesrhaying.. hardened surfacel layers as well as tubular structures, and segments of tubular structuresif, being desirable that such hardened layers be givh an initial trapped'tension stress so that the ultimate strength of the combined hardened and unhardened layers of the plate may be increased. Where a tube is employed however. the important advantage exists in that the tube resists distortion due to the stresses, whereas a strip or segmental segment would tend to bow when not loaded.

While one method has been disclosed for effecting the stresses described, and which mode is believed to be superior to any other method which might be employed for the purpose, it is to be understood that under some circumstances the internal layer may be heated by flame heating or the internal layer may be expanded through the use of high internal pressures. The use of such flame heating, however, has the disadvantage that the heat must be conducted from the surface into the structure instead of being inducted within the structure. The speed of heating is thus re-` duced and control is made more difcult. With if the yield strength of the straining" of the limits of the regard to the application of pressure to expand the internal layer and carry the external layers beyond the yield point, the danger exists that the internal layer may itself be carried beyond the tension yield point and be ruptured il care is not With electromagnetic induction heat- A appear that the internal layer is in elTect temporarily increased in its natural length and thus such danger does not exist and the stress is always compressive until the operation is completed.

While it is contemplated that through the treatment of the present invention the internal layer shall actually be placed under tension, it will appear obvious that any reduction in a residual compressive stress actually present in the internal layer would improve the strength of the tube against internal loading, this being true wherever the internal layer has a yield strength greater than the external layers and especially so when the ratio of these yield strengths is in the order of three, four or ve to one.

The tension and compressive stresses produced by this method may be limited to axial or to circumferential'str'esses if desired by merely conproceslsifovaxample if it is desired to rearrange this altialtress onlyE the piece may be const;"airreasl'iyjgiterne..the` same within a. rigid shell with the proper, diameter to retain the tube Eagainst expansion radially. In order to restrict the stresses to circumferential stresses the axial may be rigidly constrained Also by mechanically constraining the piece either circumferentially or axially beyond a set limit, the circumference or tension stresses may be completely eliminated while at the same time the tension and compressive stresses are rearranged as described t0 increase the axial strength or internal load strength, respectively. i

Although the invention has been illustrated and described in general in connection with a specic embodiment, it is to be understood that the same is not limited thereto but may be practiced in various modified forms and Ways. As many changes in the procedure and in the structure evolved therefrom may be made without deas will be apparent to those skilled in the art, reference will be had to the appended claims for a definition invention.

What is claimed is:

1. The method of enhancing the strength of a metallic tubularobject which comprises rst diflength of the tube during the process.

ferentially heating and quenching the object to stressing the momentary drawing temperature and with suol rapidity as to preclude the transfer of heat from the internal to the external zone prior to stressing the external zone beyond its and finally cooling the object, whereby upon contraction of the internal zone the external and internal zones will be placed under and tension stresses respectively.

2. The method of enhancing the strength of a tubular metallic object; which comprises differentially heatingand quenching the object to provide an internal hardened zone under residual compression stress and an external materially less hard zone under residual tension stress, and then simultaneously expanding and tension stressing the external zone beyond its elastic limit to entially heating and quenching the object to provide an internal hardened zone under residual compression stress and an external materially less hard zone under residual tension stress, and then simultaneously expanding and tension stressing the external zone beyond its elastic limit to give it a permanent set by momentarily thermally expanding the internal zone with such rapidity as to substantially prevent the transfer of heat from the internal zone to the external zone prior to the stressing of the external zone beyond its elastic limit, the ratio of the thickness of the internal zone to the thickness of the external zone being greater than the ratio of the yield strength of the external zone to the yield strength of the internal zone at the expanding temperature, whereby upon cooling of the internal zone the external zone will be placed under compression stress and the internal zone will be placed under tension stress.

4. The method of enhancing the strength oi.' a metallic tubular workpiece which comprises differentially heating and quenching the workpiece to provide a hardened internal surface layer of metal of uniform thickness under residual compressive stress and a materially less hardened remainder layer of metal, and in which the ratio oi the thicknesses respectively of said hardened and less hardened layers is greater than the ratio of the yield strengths respectively o1 said less hardened and hardened layers of metal, and then expanding the hardened layer of metal an amount sufficient to stress the less hardened layer beyond its elastic limit to give it a permanent set while retaining the internal layer in such condition that upon termination of the expansion thereof it 'will contract and place said less hardened layer under compression stress and said hardened layer under tension stress.

HOWARD E. SOMES. 

